synthesis and characterization of sapo-34/6fda-durene
TRANSCRIPT
i
Synthesis and Characterization of SAPO-34/6FDA-Durene Mixed Matrix
Membrane for CO2 Capture
by
Chua Lin Kiat
14384
Dissertation submitted in partial fulfillment of
the requirements for the
Bachelor of Engineering (Hons)
(Chemical Engineering)
SEPTEMBER 2014
Universiti Teknologi PETRONAS
Bandar Seri Iskandar
31750 Tronoh
Perak Darul Ridzuan
ii
CERTIFICATION OF APPROVAL
Synthesis and Characterization of SAPO-34/6FDA-Durene Mixed Matrix
Membrane for CO2 Capture
by
Chua Lin Kiat
14384
A project dissertation submitted to the
Chemical Engineering Programme
Universiti Teknologi PETRONAS
in partial fulfillment of the requirement for the
BACHELOR OF ENGINEERING (Hons)
(CHEMICAL ENGINEERING)
Approved by,
_____________________
(Dr. Yeong Yin Fong)
UNIVERSITI TEKNOLOGI PETRONAS
TRONOH, PERAK
September 2014
iii
CERTIFICATION OF ORIGINALITY
This is to certify that I am responsible for the work submitted in this project, that the
original work is my own except as specified in the references and acknowledgements,
and that the original work contained herein have not been undertaken or done by
unspecified sources or persons.
_____________________
CHUA LIN KIAT
iv
ABSTRACT
Membrane separation has become a promising technology in CO2 removal from natural
gas sweetening recently. In the present research, a series of SAPO-34/6FDA-durene
mixed matrix membranes (MMMs) were developed to remove the CO2 from CH4. The
MMMs were fabricated by incorporating different compositions of SAPO-34 fillers into
6FDA-durene polymer matrix. SAPO-34 fillers modified with (3-Aminopropyl)-
triethoxysilane (APTES) were synthesized and incorporated into 6FDA-durene polymer
matrix in order to study their effects on the membrane defects, as well as their effects on
the performances of the resulting MMMs towards the gas separation. The resulting
SAPO-34 and silane-modified SAPO-34 were characterized by using X-ray Diffraction
(XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron
Microscopy (SEM), whereas all the fabricated MMMs were characterized by using
Energy Dispersive X-ray (EDX) and SEM. The performances of the membranes in
CO2/CH4 separation were tested by using CO2 membrane cell filter test rig (CO2MCEF).
The morphology of the silane-modified MMMs showed the improvement on the
compatibility between the polymeric and inorganic phases. EDX results showed that the
inorganic SAPO-34 particles were evenly distributed in the polymer matrix and no
phase separation was found. However, all MMMs showed lower separation performance
compared to pure 6FDA-durene membrane mainly due to large inorganic particles size,
moisture contact, poor interfacial adhesion and polymer matrix rigidification.
v
ACKNOWLEDGEMENT
First and foremost, I would like to express my deepest gratitude to my supervisor, Dr.
Yeong Yin Fong for her guidance and persistent help throughout the period of
completing this final year project. Her endless support, guidance and experience sharing
had greatly increased my understanding, knowledge and interest in the area of this
project. Besides, she is always willing to share her personal experience and advices, so
that I can always have a clear goal and strive for the best to achieve my aim.
I would like to extend my appreciation and gratitude to Miss Norwahyu Jusoh and Miss
Lai Li Sze, research officers of Research Centre for CO2 Capture (RCCO2C) for their
kind help, cooperation and knowledge sharing. This project would not be accomplished
smoothly without their assistance and toleration.
In addition, special thanks to the laboratory technicians in Chemical Engineering
Department for their effort to assist and help me in the laboratory.
I would also like to thank my family and friends for their support and encouragement
throughout the period of project completion.
Last but not least, I would like to extend my sincere thanks to those who directly or
indirectly involved in this project. It would not have been possible without the kind
support and help of many individuals from my supervisor to the assistants in the
laboratory.
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TABLE OF CONTENTS
CERTIFICATION OF APPROVAL…………………………………………………ii
CERTIFICATION OF ORIGINALITY……………………………………………iii
ABSTRACT…………………………………………………………………………….iv
ACKNOWLEDGEMENT……………………………………………………………v
LIST OF FIGURES…………………………………………………………………viii
LIST OF TABLES……………………………………………………………………x
CHAPTER 1: INTRODUCTION…………………………………………………….1
1.1 Background of Study…………………………………………….1
1.2 Problem Statement……………………………………………….2
1.3 Objectives & Scopes……………………………………………..4
1.4 Relevancy & Feasibility………………………………………….4
CHAPTER 2: LITERATURE REVIEW…………………………………………….5
2.1 Carbon Dioxide/Natural Gas Separation Technology……………5
2.2 Polymeric Membrane…………………………………………….6
2.3 Inorganic Membrane……………………………………………10
2.4 Mixed Matrix Membrane……………………………………….13
2.4.1 Materials Selection for Development of Mixed Matrix
Membrane.…………………………………………….17
2.4.1.1 SAPO-34………………………………………17
2.4.1.2 6FDA-Durene…………………………………18
2.4.2 Challenges in Mixed Matrix Membrane Fabrication…19
CHAPTER 3: METHODOLOGY…………………………………………………22
3.1 Flow Chart of Research Methodology…………………………22
3.2 Materials……………...…………………………………………23
3.3 Equipments……………………………………………………24
3.4 Experimental Procedure………………………………………25
vii
3.4.1 Synthesis of 6FDA-Durene Polymer…………………25
3.4.2 Synthesis of SAPO-34 Crystals………………………25
3.4.3 Preparation of Silane-Modified SAPO-34 Crystals…….26
3.4.4 Preparation of 6FDA-Durene Dense Film.…………….26
3.4.5 Preparation of SAPO-34/6FDA-Durene and Silane-
Modified SAPO-34/6FDA-Durene Mixed Matrix
Membrane………………………………………………27
3.4.6 Characterization of Mixed Matrix Membrane.…………28
3.4.7 Gas Permeability and Selectivity Test………………….28
3.5 Project Activities and Key Milestones………………………….30
3.6 Gantt Chart……………………………………………………30
CHAPTER 4: RESULTS AND DISCUSSION……………………………………32
4.1 6FDA-Durene Polymer…………………………………………32
4.2 SAPO-34 Crystals………………………………………………34
4.3 Silane-Modified SAPO-34 Crystals…………………………….36
4.4 Mixed Matrix Membranes……………………………………37
4.5 Characterization of Mixed Matrix Membrane………………….40
4.5.1 Scanning Electron Microscopy…………………………40
4.5.2 Energy Dispersive X-ray………………………………43
4.6 Gas Separation Performance……………………………………47
CHAPTER 5: CONCLUSION AND RECOMMENDATION……………………50
5.1 Conclusion……………………………………………………50
5.2 Recommendation……………………………………………….51
REFERENCES………………………………………………………………………53
viii
LIST OF FIGURES
Figure 1 Trade-off curve between selectivity and permeability of CO2/CH4 gas
separation separation…………………………………………………………………….8
Figure 2 Framework structure of SAPO-34………………………………………….17
Figure 3 Synthesis scheme of 6FDA-durene…………………………………………19
Figure 4 Schematic diagram of (A) ideal morphology of MMM and (B) interface
voids between inorganic filler and polymer matrix………………………20
Figure 5 Flow chart of overall research methodology………………………………22
Figure 6 CO2 membrane cell filter test rig (CO2MCEF).…………………………….29
Figure 7 Purified 6FDA-dianhydride monomers…………………………………….32
Figure 8 Purified durene-diamine monomers………………………………………...32
Figure 9 6FDA-durene polyimide after synthesis……………………………………33
Figure 10 6FDA-durene polyimide after drying………………………………………33
Figure 11 SAPO-34 crystals before drying……………………………………………34
Figure 12 Final products of SAPO-34 crystals after calcination………………………35
Figure 13 SEM image of SAPO-34 crystals. The scale bar (8 µm) is represented by 4
grids…………………………………………………………………………35
Figure 14 XRD pattern of synthesized SAPO-34 crystals…………………………….36
Figure 15 Stirring of silane-modified SAPO-34 crystals mixture under nitrogen purge
………………………………………………………………………………36
Figure 16 FTIR spectra of SAPO-34 and silane-modified SAPO-34 crystals………37
Figure 17 Sonication of SAPO-34/6FDA-durene mixture in sonicator…………….…38
ix
Figure 18 Slow solvent evaporation of casted membrane……………………………38
Figure 19 SAPO-34/6FDA-durene mixed matrix membranes with 0, 5, 10, 15 and 20
wt% SAPO-34 loadings…………………………………………………….39
Figure 20 Silane-modified SAPO-34/6FDA-durene mixed matrix membranes with 0, 5,
10, 15 and 20 wt% silane-modified SAPO-34 loadings……………………39
Figure 21 Cross-section SEM image of pure 6FDA-durene membrane………………40
Figure 22 Comparison of cross-section SEM images of SAPO-34/6FDA-durene and
silane-modified SAPO-34/6FDA-durene MMMs. (5, 10, 15 and 20) and (S5,
S10, S15 and S20) represent the loadings of SAPO-34 and silane-modified
SAPO-34 in wt%, respectively……………………………………………41
Figure 23 EDX data of pure 6FDA-durene membrane, SAPO-34/6FDA-durene and
silane-modified SAPO-34/6FDA-durene MMM. (15) and (S15) represent the
loadings of SAPO-34 and silane-modified SAPO-34 in wt%, respectively..43
Figure 24 EDX mapping of SAPO-34/6FDA-durene MMM loaded with 15 wt%
SAPO-34……………………………………………………………………45
Figure 25 EDX mapping of silane-modified SAPO-34/6FDA-durene MMM loaded
with 15 wt% silane-modified SAPO-34…………………………………….46
Figure 26 Schematic diagram of (A) matrix rigidification and (B) plugged sieves…49
x
LIST OF TABLES
Table 1 CO2/CH4 separation performances of polymeric membranes…………….…9
Table 2 CO2/CH4 separation performances of inorganic membranes………………12
Table 3 CO2/CH4 separation performances of mixed matrix membranes……………15
Table 4 Key milestones of the research.……………………………………………30
Table 5 Gantt chart.…………………………………………………………………31
Table 6 Permeability and CO2/CH4 separation selectivity at 25oC and 5 bar………47
1
CHAPTER 1
INTRODUCTION
1.1 Background of Study
Carbon dioxide (CO2) is a greenhouse gas mainly found in the combustion product of
fossil fuels, natural gas stream, biogas and landfill gas. The purpose of removing CO2
from those gas streams, especially natural gas stream is to obtain a purified fuel with
enhanced energy content and to prevent corrosion problems in the gas transportation
system. These reasons have driven the development of CO2 gas separation process
technology. One of the separation technologies that have successfully gained the
attention is the membrane-based technology. Membrane technology has emerged as a
potential process in natural gas sweetening (CO2/CH4) apart from adsorption, absorption
and cryogenic process due to its high energy efficiency, simple in design and
construction of membrane modules and environmental compatibility (Zhang, Sunarso,
Liu, & Wang, 2013).
Natural gas sweetening is a purification process to remove the acidic gases such as CO2,
hydrogen sulfide (H2S) and sulfur dioxide (SO2) before it is compressed and delivered
for sale. The presence of CO2 in the natural gas will lower its caloric value and thus,
lower the selling value of natural gas. Moreover, the presence of acidic CO2
containments in the gas stream will corrode and damage the equipments and pipelines
with the presence of water in the transportation and storage system, and hence reduces
the process plant efficiency (Zhang et al., 2013)
Membrane-based technology uses membrane materials such as polymeric membrane,
inorganic membrane and mixed matrix membrane (MMM) to separate CO2 from natural
gas. Generally, the permeation and selectivity are the two common basic performance
characteristics of a membrane. Permeability can be defined as the ability of permeate to
pass through a membrane, while selectivity of the membrane is the ratio of permeability
of the more permeable component to that of the less permeable (Goh, Ismail, Sanip, Ng,
2
& Aziz, 2011). Higher permeability decreases the amount of membrane area used, thus
reduces the capital cost of membrane units, while higher selectivity will increase the
purity of gas product. Despite the membrane separation process is energy efficient,
operating and capital cost savings, ease of process design and operation, the
development of the high performance membrane material still possesses a big challenge
in membrane technology. It is difficult to maintain the high performance and
consistency of a membrane over a long period.
The performance of the existing membranes such as polymeric membrane is limited by
Robeson’s upper bound limit while inorganic membrane has a low mechanical
properties and high manufacturing cost (Goh et al., 2011; Zhang et al., 2013). Therefore,
MMM becomes a new class of membrane materials that is potential to enhance the
current membrane technology by overcoming the limitations of the polymeric
membrane and inorganic membrane. The incorporation of inorganic fillers in a polymer
matrix is expected to increase the permeability, selectivity, or both compared to the
current existing membrane materials.
Recent literature reviews reported that SAPO-34 promotes high CO2 selectivity, and
incorporation of SAPO-34 in polymer matrix could produce a MMM with better
CO2/CH4 gas separation (Junaidi, Khoo, Leo, & Ahmad, 2014). Therefore, in the present
research, new type of MMM namely SAPO-34/6FDA-durene is developed. Besides, a
chemical-modified SAPO-34 which can improve the interface and reduce the voids
during the MMM fabrication is synthesized and used as inorganic fillers in MMM.
Characterization and separation performances of the resulting membranes would be
studied.
1.2 Problem Statement
Even though polymeric membrane possesses advantages of mechanical properties,
reproducibility and relative economical processing capability, but this membrane
material has demonstrated a trade-off between the permeability and selectivity, which is
known as Robeson’s upper bound limit (Robeson, 2008). Robeson’s upper bound limit
3
is an inverse relationship exists between the permeability and selectivity, where more
permeable polymer is generally less selective and vice versa. In order to enhance the gas
separation performances, mixed matrix membrane (MMM) has been developed to
transcend the upper bound limit.
In MMM fabrication, inorganic fillers are normally dispersed at a nanometer level in a
polymer matrix. The resulting material possesses the favorable properties of inorganic
filler and polymeric membrane, thus its performance in gas separation could surpass the
trade-off issue. Another challenge in the development of MMM is to obtain a good
compatibility or interfacial contact between the continuous polymer matrix and
inorganic fillers. Incompatibility of polymer and filler phases during the formation of
MMM may deteriorate the gas separation performance of the resulting membrane and
leads to the void formation (Cong, Zhang, Radosz, & Shen, 2007). The formation of
these non-selective voids at the interface allows the bypassing of gases and hence,
decreases the selectivity and permeability of the resulting MMM.
The incorporation of SAPO-34 fillers in polymer matrix always leads to the poor
compatibility between the two phases (Li, Guan, Chung, & Kulprathipanja, 2006).
However, it was found that the application of silane coupling agents on the surface of
SAPO-34 zeolite can improve the interfacial strength between these two phases. The
selection of appropriate silane groups and their effect towards the separation
performance of MMM remain as new tasks to be discovered.
Besides, the loading of inorganic fillers and their particle size will affect the morphology
and performances of MMM. The synthesis of inorganic fillers in smaller size and
optimum loading of inorganic fillers in MMM can minimize the void volumes, and thus
increase the performances of the MMM.
Therefore, better understanding on the improvement and optimization of MMM process
to overcome the abovementioned problems remains as a challenging topic for future
investigation and development.
4
1.3 Objectives & Scopes
The primary purpose of this research is to synthesis a high permeability and selectivity
membrane for separation of carbon dioxide (CO2) from natural gas.
There are several objectives and scopes to be achieved in this research project. These
objectives are:
1. To synthesis 6FDA-durene polymer, SAPO-34 and silane-modified SAPO-
34 particles.
2. To fabricate mixed matrix membranes (MMMs) by incorporating different
loadings of SAPO-34 and silane-modified SAPO-34 as inorganic fillers into
6FDA-durene polymer.
3. To characterize the resulting particles and membranes by using X-ray
Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR),
Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX).
4. To study the performance of the resulting MMMs in CO2/CH4 separation.
1.4 Relevancy & Feasibility
Mixed matrix membrane (MMM) is an emerging membrane material in membrane-
based separation technology that can be used to separate carbon dioxide (CO2) from
natural gas. It is synthesized by incorporating porous inorganic fillers in a polymer
matrix. MMM is a potential material that can overcome the limitations of the polymeric
and inorganic membrane. This research is feasible within the scopes identified and the
time allocated. The first half of this research is focused on the literature study of the
related researches as well as project planning. While the second half of the research
would be focused on the experimentation and results collecting. The results collected
would then be analyzed and investigated critically.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Carbon Dioxide/Natural Gas Separation Technology
Carbon dioxide (CO2) is the main component of the greenhouse gases. The
accumulation of CO2 in the environment will lead to severe global warming issues.
Global CO2 Budget 2013 reported that global CO2 emissions achieved 36 billion metric
tons in year 2012. Human activities such as open burning and transportation will
contribute to the large amounts of CO2 emissions. CO2 can also be found in the natural
gas stream, biogas from anaerobic digestion, landfill gas, combustion product of fossil
fuel and product of coal gasification. Natural gas reserves are usually contaminated with
70% of CO2 and N2, but pipelines specifications for natural gas normally require a CO2
concentration below 2-3% (Venna & Carreon, 2011). Besides, CO2 will reduce the
heating value of natural gas, and it is acidic and corrosive with the presence of water in
the transportation and storage system. Therefore, the separation of CO2 from CH4 is very
important in many industrial processes especially in natural gas sweetening.
The conventional separation methods employed for CO2 separation process include
chemical absorption by reactive solvents, pressure swing adsorption (PSA), temperature
swing adsorption (TSA) and cryogenic separation (Basu, Khan, Cano-Odena, Liu, &
Vankelecom, 2010). The conventional separation method is operated at a high
temperature and pressure, and it yields high purity and efficiency. However, these
conventional separation methods usually involve substantially complicated equipment,
higher energy consumption and capital cost.
Therefore, as an alternative, membrane separation technology appears to be a potential
and efficient method for gas separation. It becomes an attractive separation approach
due to its fast and energy efficient process without any phase changes. Membrane
separation technology uses membrane as a thin barrier between miscible fluids to
separate a mixture, such as CO2 and natural gas mixture. During the separation process,
6
a driving force such as concentration or pressure differential is used to transport a
selective component across the membrane. Membrane is a promising material for
CO2/CH4 separation due to its advantages which include high efficiency and stability,
low operating and capital cost, less energy required, simple process design, ease of
operation as well as environmental friendly (Goh et al., 2011).
However, although membrane separation technology is promising in gas separation, it is
difficult to maintain the membrane performance and consistency in long term operation.
Most membranes do not have the resilience in practical industry conditions (Zhang et
al., 2013). A high performance membrane used in natural gas sweetening should possess
good chemical, thermal and mechanical stability, high CO2 permeability, and high
selectivity towards CO2 and natural gas separation (Freemantle, 2005).
Membrane materials are classified into polymeric membrane, inorganic membrane and
mixed matrix membrane (MMM). They would be discussed in details in the following
sections.
2.2 Polymeric Membrane
Polymeric membrane, particularly glassy polymer is currently the dominant material for
gas separation process such as natural gas sweetening, landfill gas recovery, hydrogen
recovery and purification, flue gas separation and air separation (Bastani, Esmeili, &
Asadollahi, 2013).This is because polymeric membrane possesses the advantages of
mechanical properties, reproducibility, flexibility to be processed into different modules
and relative economic processing capability.
Polymeric membranes are classified into porous and non-porous membrane. Porous
membrane has rigid, highly voided structure with randomly distributed and
interconnected pores (Goh et al., 2011). The separation in polymeric membrane is
dependent on the molecular size and pore size distribution. Non-porous membranes or
dense membranes consist of a dense film where the permeate molecules are absorbed
and diffused through the membrane matrix under the driving force of a concentration,
7
pressure, or electrical potential gradient (Goh et al., 2011). The mechanism of the gas
transport is determined by the diffusivity and solubility of the permeant molecules in the
membrane material. The transport of gas molecules in polymeric membrane is usually
based on solution-diffusion mechanism. Polymer materials such as polyimide (PI),
cellulose acetate (CA), polysulfone (PSF), polyethersulfone (PES) and polycarbonates
(PC) have been used to fabricate the polymeric membranes for gas separation (Zhang et
al., 2013). Polymer materials such as PI and CA have been commercially used in
CO2/CH4 separation (Han & Lee, 2011). Other glassy polymers such as PSF,
polyethermide (PEI), polymer of intrinsic microporosity (PIM) and Matrimid are still
undergoing tremendous researches to improve the membrane performance (Zhang et al,.
2013). Table 1 shows the CO2/CH4 separation performances of polymeric membranes.
Even though polymeric membranes dominate the current CO2 separation membrane
technologies due to their low cost, outstanding mechanical stability and other
advantages, but they still suffer from either low permeability or low selectivity. The
performance of polymeric membrane is governed by the Robeson’s upper bound limit
(Robeson, 2008). Robeson’s upper bound limit is an inverse relationship exists between
the permeability and selectivity. High permeability polymeric membrane always has a
low selectivity and vice versa. Figure 1 shows the trade-off of CO2 permeability versus
CO2/CH4 selectivity for gas separation. It can be seen that only minority of the
membranes fall above or close to the curve. Besides, commercial polymeric membrane
also suffers from an issue of plasticization, where the CO2 concentration in the feed
stream is high (Zhang et al., 2013). CO2-induced plasticization is a phenomenon where
the CO2 permeability increases as a function of pressure while the selectivity decreases
(Ismail & Lorna, 2002).
In order to overcome the inherent limitations of polymeric membrane, researches are
underway for alternative membrane materials for effective separation of CO2 from
natural gas.
8
Figure 1: Trade-off curve between selectivity and permeability of CO2/CH4 gas
separation (Robeson, 2008; Zhang et al., 2013)
2008
1991
9
Table 1: CO2/CH4 separation performances of polymeric membranes
Membrane Material Pressure
(bar)
Temperature
(oC)
CO2 Permeability CO2/CH4
Selectivity Reference
Value Unit
6FDA-BAPAF 30 21 24.6 GPU 22.8 Kim, Park, So, Ahn, & Moon (2003)
6FDA-DAP 30 21 38.6 GPU 77.8 Kim et al. (2003)
6FDA-DABA 30 21 26.3 GPU 47 Kim et al. (2003)
6FDA-1, 5-NDA 10 35 22.6 Barrer 49 Chan, Chung, Liu, & Wang (2003)
6FDA-pPDA 10 35 15.3 Barrer - Lin, Vora, & Chung (2000)
6FDA-Durene 10 35 458 Barrer 16.1 Liu, Chng, Chung, & Wang (2003)
6FDA-Durene/mPDA (50:50) 10 35 84.6 Barrer 29.9 Liu et al. (2003)
6FDA-Durene/NDA (75:25) 10 35 274 Barrer 21.1 Chan et al. (2003)
6FDA-Durene/pPDA (80:20) 10 35 230 Barrer - Lin et al. (2000)
6FDA-DAM 2 25 390 Barrer 24 Bae et al. (2010)
Matrimid®5218 34.5 35 10 Barrer 35.5 Vu, Koros, & Miller (2003)
Matrimid®5218 2.66 35 9.52 Barrer 39.8 Ordoñez, Balkus Jr, Ferraris, &
Musselman (2010)
10
2.3 Inorganic Membrane
Inorganic membrane is increasingly being investigated due to its attractive
characteristics and advantages over the polymeric membrane for many gas separation
processes. Inorganic membrane can be categorized as porous and non-porous. Porous
inorganic membranes such as zeolite and carbon molecular sieve (CMS) are favorable
for CO2/CH4 gas separation due to their superior selectivity to polymeric membrane.
According to Carreon, Li, Falconer, & Noble (2008), SAPO-34 zeolite membrane on
porous α-alumina supports exhibited the CO2/CH4 selectivity range from 86 to 171 with
CO2 permeability of 20,000-40,000 Barrer.
Non-porous inorganic membranes are attractive for high temperature (above 400oC)
application only when the amount of ionic defects in these membranes are enough to
allow the appreciable ionic movement and, thus, high conductivity. Non-porous
inorganic membrane is represented by a dual phase membrane containing mixed
conducting oxide ceramic (MCOC) and molten carbonate phases (Anderson & Lin,
2010; Li, Rui, Xia, Anderson, & Lin, 2009; Rui, Anderson, Lin & Li, 2009).
The ability of inorganic membrane to withstand high temperature for long time and the
resistance to harsh operating environment have brought it to be a promising material for
application in membrane reactors in industrial processes. Moreover, inorganic
membranes also have a higher throughput and longer lifespan compared to polymeric
membrane.
Different inorganic membrane will exhibit different gas separation performance. It is
selective to certain gas pair’s separation. Table 2 shows the CO2/CH4 gas separation
performances of inorganic membranes. The unique characteristic of inorganic
membrane makes it demonstrates high gas flux and selectivity. Nevertheless, inorganic
membrane is limited to the severe operating conditions due to the high manufacturing
cost and low reproducibility. Besides, inorganic membrane is also facing the drawback
of poor mechanical properties. It is brittle and usually with low surface-to-volume ratio
(Goh et al., 2011). Furthermore, the presence of other gas components such as water and
hydrogen sulfide (H2S) may cause negative effects to the CO2 separation performance of
11
the membrane (Li, Alvarado, Noble, & Falconer, 2005). Therefore, inorganic membrane
is still not a commercialized material to use in industrial applications.
12
Table 2: CO2/CH4 separation performances of inorganic membranes
Membrane Material Pressure
(bar)
Temperature
(oC)
CO2 Permeability CO2/CH4
Selectivity Reference
Value Unit
SAPO-34 zeolite (on porous
stainless steel tube) 2.2 22 1045.9 GPU 120 Li, Falconer, & Noble (2006)
KY zeolite (on porous
alumina tube) 1 30 2091.8 GPU 40
Hasegawa, Tanaka, Jeong,
Kusakabe, & Morooka (2002)
DDR zeolite (on porous
alumina disk) 5 28 7.0 x 10
-8 mol/(m
2sPa) 220
Tomita, Nakayama, & Sakai
(2004)
Zeolite-T 5 30 0.70 x 10-8
mol/(m2sPa) 70.8
Mirfendereski, Mazaheri,
Sadrzadeh, & Mohammadi
(2008)
Zeolite-T 1 35 4.60 x 10-8
mol/(m2sPa) 400 Cui, Kita, & Okamoto (2004)
ZIF-8 (on porous α-
alumina) 1.4 22 1.69 x 10
-5 mol/(m
2sPa) 7 Venna & Carreon (2009)
CMS 550-2 3.4 35 1250 Barrer 63 Vu et al. (2003)
CMS 800-2 3.4 35 43.5 Barrer 200 Vu et al. (2003)
Silicate-1
1.01 30 589.84 GPU 4.3 Zhu, Hrabanek, Gora, Kap teija,
& Moulijn (2006)
13
2.4 Mixed Matrix Membrane
Due to the limitations of the polymeric membrane and inorganic membrane, a new
membrane material which is known as mixed-matrix membrane (MMM) have been
introduced to overcome the restrictions of the polymeric membrane and inorganic
membrane. This emerging membrane material is expected to enhance the existing
membrane-based separation technology by overcoming the polymer-inorganic phase
separation problems. Besides, the MMM produced should also improve the
compatibility between the polymer matrix and inorganic fillers as well as defect-free.
MMM is fabricated by incorporating the inorganic fillers as dispersed phase in a
continuous polymer matrix:
Inorganic fillers + Polymer matrix Mixed matrix membrane
Different combinations of inorganic phase in polymer matrix will synthesis the MMM
with different membrane performances. The choice of the polymer matrix and the
inorganic filler particles with different loadings are the two important parameters that
will affect the morphology and performance of the MMM. In order to choose the most
suitable continuous polymer phase and dispersed inorganic phase in the MMM
fabrication, the gas transport mechanism and the gas component transporting through
the membrane should take into consideration (Chung, Jiang, Li, & Kulprathipanja,
2007). Table 3 shows the CO2/CH4 gas separation performances of MMM.
The incorporation of inorganic fillers in polymer matrix should synthesis an ideal
membrane material which possesses high permeability, high selectivity and high
chemical, thermal and mechanical stability (Freemantle, 2005). The effective
permeability of the MMM is calculated by using the Maxwell model, as shown in
equation 1 (Noble, 2011). This equation is valid for spherical particles in dilute
suspensions where the interaction between the particles is negligible.
( )
( ) ---------- (1)
14
is the effective permeability of the MMM, is the permeability of the continuous
polymer phase, is the permeability of the dispersed inorganic phase and is the
volume fraction of the dispersed phase.
15
Table 3: CO2/CH4 separation performances of mixed matrix membranes
Mixed Matrix Membrane (MMM)
Composition Pressure
(bar)
Temperature
(oC)
CO2
Permeability CO2/CH4
Selectivity Reference
Polymer Inorganic Filler Value Unit
6FDA-DAM ZIF-90 (15 wt%) 2 25 720 Barrer 37 Bae et al. (2010)
6FDA-DAM MOFs (0%) 2 25 390 Barrer 24 Bae et al. (2010)
6FDA-ODA UiO-66 10 35 50.4 Barrer 46.1 Nik, Chen, & Kaliaguine
(2012)
6FDA-ODA Al-MIL-53-NH2 (32
wt%) 10 35 14.5 Barrer 80
Chen, Vinh-Thang,
Rodrigue, & Kaliaguine
(2012)
6FDA-Durene ZIF-8 (33.3 wt%) 3.5 35 1552.9 Barrer - Wijenyaka et al. (2013)
PC Zeolite 4A - - 4.6 Barrer 51.8 Cite in Goh et al. (2011)
PES Zeolite 4A - - 6.7 Barrer 28.7 Cite in Goh et al. (2011)
PES A zeolite with silver ion
exchange (50 wt%) 10 35 1.2 Barrer 44 Cite in Goh et al. (2011)
16
PDMC SSZ-13 zeolite - - 88.6 Barrer 41.9 Cite in Goh et al. (2011)
PI MWCNTs (1 wt%) - - 14.3 Barrer 10 Cite in Goh et al. (2011)
PIM-1 ZIF-8 (43 vol%) 1 23 6300 Barrer 14.7 Bushell et al. (2013)
PSF SAPO-34E 4.48 25 706 GPU 31 Junaidi, Khoo, Leo, &
Ahmad (2014)
Matrimid®9725 ZIF-8 (30 wt%) 10 35 22 GPU 20 Basu, Cano-Odena, &
Vankelecom (2011)
Matrimid®5218 ZIF-8 (30 wt%) 4 22 28.72 Barrer 24.9 Song et al. (2012)
Matrimid®5218 ZIF-8 (40 wt%) 2.66 35 24.55 Barrer 27.84 Ordoñez et al. (2010)
Ultem®1000 CMS 800-2 (35 vol%) 3.4 35 4.48 Barrer 53.7 Vu et al. (2003)
17
2.4.1 Materials Selection for Development of Mixed Matrix Membrane
In this research, SAPO-34 crystals and 6FDA-durene are selected as the inorganic fillers
and polymer matrix, respectively to synthesis the mixed matrix membrane (MMM).
SAPO-34 particles are incorporated into 6FDA-durene polymer to fabricate the MMM
that can enhance the CO2/CH4 gas separation process. Characterization and the
performances of the resulting MMM are studied. The literatures and reasons for
investigating these combinations are discussed in the following sections.
2.4.1.1 SAPO-34
SAPO-34 zeolite is a silicoaluminophosphate molecular sieve with the composition
(SixAlyPz)O2, where x = 0.01-0.98, y = 0.01-0.60 and z = 0.01-0.52. Figure 2 shows the
framework structure of SAPO-34. Li, Falconer, & Noble (2006) reported that SAPO-34
molecular sieve has a 0.38 nm framework pore diameter; which is similar in size to CH4
but larger than CO2. SAPO-34 has high CO2/CH4 selectivity due to its remarkable
molecular sieving, combination of differences in diffusivity and enhanced competitive
CO2 adsorption properties (Venna & Carreon, 2011; Li et al., 2006). Adsorption
isotherms showed that CO2 adsorbs more strongly than CH4 on SAPO-34 crystals (Li et
al., 2006). Li et al. (2006) reported that SAPO-34 membrane showed separation
selectivity as high as 95 and CO2 permeability of 1433 Barrer. Due to these advantages,
SAPO-34 receives great attention as inorganic fillers in mixed matrix membrane
(MMM) fabrication.
Figure 2: Framework structure of SAPO-34 (Venna & Carreon, 2011)
0.38 nm
18
However, it was found that there is a challenge in incorporating SAPO-34 fillers in the
polymer phase, which is the poor compatibility between these two phases. The recent
research found that there are some methods that can improve the interfacial strength
between these two phases in order to enhance their membrane performance. One of
them is the silane (chemical) modification of the surface of SAPO-34 zeolite with silane
coupling agent, such as (3-Aminopropyl)-triethoxysilane (APTES). Silane coupling
agent is a silicon-based chemical which contains two types of reactive groups, namely
organic and inorganic groups in the same molecule (Junaidi, Khoo, Leo, & Ahmad,
2014). By introducing the silane coupling agent, it may modify the surface properties of
SAPO-34 zeolite from hydrophilic to hydrophobic, and increase SAPO-34 affinity to the
functional groups of the polymer matrix (Li et al, 2006).
However, even though the application of silane coupling agent in the inorganic phase
can improve the interface and reduce the voids in the membrane, but it may increase the
gas transport resistance across the membrane, and decrease the permeability and
selectivity of the membrane if an improper silane coupling agent is selected. Therefore
in the present research, APTES is introduced on SAPO-34 zeolite surface to study their
effects on the separation performance of mixed matrix membrane (MMM). APTES can
react with hydroxyl groups, amino groups and other functional groups from zeolite and
polymer matrix to improve the compatibility between the two phases (Zhang et al.,
2013). Hibshman et al. (2003) found that the organosilicate can be covalently bonded to
the polyimide matrix.
In the present research, SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-
durene MMMs are fabricated to compare their performances towards the CO2/CH4 gas
separation.
2.4.1.2 6FDA-Durene
6FDA-durene is a type of aromatic Polyimide (PI) materials. Referring to Table 1,
6FDA-durene shows the highest CO2 permeability, which is 458 Barrer. 6FDA-durene
becomes an ideal polymer phase in mixed matrix membrane fabrication due to its
excellent thermal and mechanical properties. It shows a good intrinsic gas separation
19
performance as compared to other 6FDA-based polyimides (Liu, Chng, Chung, &
Wang, 2003).
6FDA-durene is synthesized by using the chemical imidization method. Figure 3 shows
the synthesis scheme and the molecular structure of 6FDA-durene that is used to
fabricate the mixed matrix membrane (MMM) for CO2/CH4 gas separation.
Figure 3: Synthesis scheme of 6FDA-durene (Wijenayake et al., 2013)
The four methyl groups that are present in the diamine moiety increase the gas
diffusivity and solubility, and inhibit the dense chain packing, thus increase the
membrane permeability (Chan, Chung, Liu, & Wang, 2003; Liu, Wang, Liu, Chng, &
Chung, 2001).
Due to these advantages, 6FDA-durene is selected as the polymer phase in the MMM
fabrication. The synthesis procedure of 6FDA-durene is prepared in the methodology
sections.
2.4.2 Challenges in Mixed Matrix Membrane Fabrication
Even though mixed matrix membrane (MMM) possesses high performances in gas
separation technology, but there are some challenges in the development of MMM that
will reduce its performances.
6FDA-durene
20
One of the major challenges in MMM fabrication is the dispersibility of inorganic fillers
in the polymer matrix. In ideal situation, the dispersed inorganic phase should evenly
distribute in the continuous polymer phase. Overloading of inorganic fillers will cause
the agglomeration of inorganic fillers in the MMM and hence, reduces the gas
permeability. Therefore, an optimum loading of inorganic fillers is important to
maintain the morphology of the membrane and to avoid the phase separation between
the polymer phase and the inorganic phase.
Another problem that is faced by MMM is the incompatibility between the inorganic
fillers and the polymer matrix. This condition will create the separate phases in MMM
and deteriorate the gas performance of the membrane. The improper/poor interfacial
contact between the dispersed phase and the continuous phase will lead to the formation
of void volume as illustrated in Figure 4. The formation of non-selective voids at the
interface will allow the bypassing of gases, which in turn decreases the membrane
selectivity (Goh et al., 2011). One of the effective methods that can be used to eliminate
the interfacial voids is by applying the silane coupling agent into the inorganic phase
prior to the fabrication of MMM. Silanes can react with hydroxyl groups, amino groups
and other functional groups from zeolite and/or the polymer matrix to improve the
compatibility between the phase boundaries (Zhang et al., 2013).
Figure 4: Schematic diagram of (A) ideal morphology of MMM and (B) interface voids
between inorganic filler and polymer matrix
The next factor that will affect the performance of the MMM is the particle size of the
inorganic fillers (Noble, 2011). The average filler particle size should be in nano-scale
Polymer matrix Polymer matrix
Filler Filler
Interface voids Ideal morphology
A B
21
regime and uniform in shape. This is because smaller particles possess higher surface
area/volume ratios. It will enhance the mass transfer of gas molecules across the
membrane and hence, increases the performance of the membrane.
22
CHAPTER 3
METHODOLOGY
3.1 Flow Chart of Research Methodology
The overall research methodology is shown in Figure 5.
Figure 5: Flow chart of overall research methodology
Synthesis of 6FDA-durene polymer
Synthesis of SAPO-34 crystals
Fabrication of SAPO-34/6FDA-durene mixed matrix membrane (MMM)
Preparation of silane-modified SAPO-34 crystals
Fabrication of silane-modified SAPO-34/6FDA-durene MMM
Data evaluation and report writing
Gas permeability and selectivity test
Characterization of MMM
23
3.2 Materials
The materials and chemicals used in the present research are as follows:
4, 4’-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA, 99% purity,
Sigma-Aldrich) monomers are purified by vacuum sublimation prior to use.
2, 3, 5, 6-Tetramethyl-p-phenylenediamine (durene-diamine, 99% trace metals
basis, Sigma-Aldrich) monomers are purified by re-crystallization in methanol
prior to use.
The solvent, N-methyl-2-pyrolidone (NMP) is purified by using a rotary
evaporator.
Propionic anhydride (PA, 98% purity, Merck) and triethylamine (TEA, 99%
purity, Merck) are used as received.
Methanol ( 99.9% purity, Merck) and dichloromethane ( 99.8% purity, Sigma-
Aldrich) solvent are used as received.
Phosphoric acid (H3PO4, 85% aqueous solution, Merck).
Aluminium triisopropylate (C9H21AlO3, 98% purity, Merck).
Tetraethylammonium hydroxide (TEAOH, ~40% aqueous solution, Sigma
Aldrich) template solution.
Tetraethyl-orthosilicate (TEOS, 99% purity, Merck).
Toluene (99.8% purity, Sigma-Aldrich).
The silane coupling agent, (3-Aminopropyl)-triethoxysilane (APTES, 98%
purity, Sigma-Aldrich) is used without further purification.
24
3.3 Equipments
The equipments that are used in the present research are as follows:
Vacuum sublimation equipment is used to purify the 6FDA monomers.
Rotary evaporator is used to purify the N-methyl-2-pyrolidone (NMP).
Vacuum oven is used to dry the polymer precipitates, SAPO-34 crystals, cast
films of 6FDA-durene and mixed matrix membrane (MMM). It is also used to
anneal the dense films of 6FDA-durene and MMM.
Teflon-lined synthesis reactor is used to carry out the hydrothermal synthesis
of SAPO-34 crystals.
Furnace is used to calcine the SAPO-34 crystals in air condition.
Sonicator is used to evenly disperse the SAPO-34 crystals in the polymer
solution.
X-ray Diffraction (XRD) is used to verify the presence of SAPO-34 in the
resulting MMM.
Fourier Transform Infrared Spectroscopy (FTIR) spectrometer is used to
determine the functional groups and silane-modified grafting present in SAPO-
34 crystals.
Scanning Electron Microscope (SEM) is used to study the morphology of the
MMM.
Energy Dispersive X-ray (EDX) is used to study the dispersion of fillers in the
MMM.
CO2 membrane cell filter test rig (CO2MCEF) is used to test the performance
of the MMM in CO2/CH4 separation.
25
3.4 Experimental Procedure
3.4.1 Synthesis of 6FDA-Durene Polymer
The synthesis of 6FDA-durene polymer was carried out by using the chemical
imidization method (Wijenayake et al., 2013; Liu et al., 2001). The experimental
procedure was as follows:
1. Durene-diamine monomers were dissolved in purified NMP. Nitrogen purge was
applied throughout to reduce the side reactions that would occur when the
solution was exposed to the air which contains oxygen and water moisture.
2. Equal-mole of 6FDA monomers were added to the solution to obtain a 21 wt%
concentration.
3. The mixture was stirred for 24 hours at room temperature under nitrogen purge
to obtain polyamic acid (PAA) solution.
4. Propionic anhydride and triethylamine were added slowly to the PAA solution
for chemical imidization, with the mole ratio of propionic anhydride/
triethylamine to 6FDA of 4:1. The mixture was stirred for 24 hours at room
temperature under nitrogen purge to form 6FDA-durene polyimides.
5. The polyimides were precipitated in methanol and then washed with methanol
for several times.
6. The wash solution was filtered and the collected polymer precipitates were dried
at 150°C in vacuum oven for 24 hours to remove any wash solution residue.
3.4.2 Synthesis of SAPO-34 Crystals
SAPO-34 crystals were synthesized by hydrothermal synthesis method reported by
Askari & Halladj (2012) as follows:
1. A gel with molar composition of Al2O3: P2O5: 0.6 SiO2: 2 TEAOH: 70 H2O was
prepared by mixing C9H21AlO3, TEAOH and deionized water. The mixture was
stirred for an hour at room temperature to form homogeneous solution.
26
2. Silica source (TEOS) was added into the mixture and stirred for 2 hours. After
that, with continuous stirring, H3PO4 was added drop wise to the solution. The
solution was further stirred for an hour.
3. The gel solution was then transferred into the Teflon-lined synthesis reactor for
hydrothermal growth. The hydrothermal synthesis was carried out at 200oC for
24 hours.
4. After synthesis, the solid products were recovered and washed three times by
centrifuging with deionized water.
5. The products were dried in oven at 110oC overnight.
6. The as-synthesized crystals were calcined in furnace at 550oC in air for 6 hours
to remove the organic template molecules.
3.4.3 Preparation of Silane-Modified SAPO-34 Crystals
The synthesis of silane-modified SAPO-34 crystals was carried out by using the
procedure as follows (Li et al., 2006):
1. A mixture of 120 ml toluene, 4.8 ml silane coupling agent (APTES) and 0.6 g
SAPO-34 crystals was stirred for 24 hours at room temperature under nitrogen
purge.
2. SAPO-34 crystals were filtered with toluene and then washed three times by
centrifuging with methanol to remove the unreacted silane.
3. The silane-modified SAPO-34 crystals were dried in oven at 110oC overnight.
3.4.4 Preparation of 6FDA-Durene Dense Film
6FDA-durene dense film was prepared as follows (Liu et al., 2002):
1. A 3% w/v solution of polymer in dichloromethane was prepared and filtered
through a 1 μm filter to remove the non-dissolved materials and dust particles.
2. The solution was stirred for 1 hour and then slowly poured into a Petri dish on a
leveled clean glass plate.
3. The Petri dish was covered with a piece of glass and a small gap was kept for
slow solvent evaporation at room temperature overnight.
27
4. The cast film was dried in an oven at 60°C for 24 hours without vacuum and
another 24 hours with vacuum. Then, the oven temperature was increased from
60 to 250°C at a heating rate of 25°C/hour.
5. The dense film was annealed at 250°C for 24 hours and then slowly cooled down
in the oven.
3.4.5 Preparation of SAPO-34/6FDA-Durene and Silane-Modified SAPO-
34/6FDA-Durene Mixed Matrix Membrane
The mixed matrix membrane (MMM) with inorganic fillers loadings was prepared by
using the method as follows (Wijenayake et al. 2013):
1. SAPO-34 dispersion and polymer solution were prepared separately. SAPO-34
crystals of 5, 10, 15 and 20 wt% were added to the dichloromethane in separate
vials, and stirred and sonicated for 2 hours (alternating 30 minutes stirring
followed by 30 minutes sonication) to produce SAPO-34 dispersion.
2. 6FDA-durene polymer was added to the dichloromethane and stirred for 1 hour
to produce polymer solution.
3. 10% of the polymer solution was added to the SAPO-34 dispersion (priming).
The mixture was stirred and sonicated for 2 hours.
4. The remaining polymer solution was added and the mixture was further stirred
and sonicated for 2 hours, and finally stirred for 2 hours.
5. The mixture was casted on a Petri dish and the Petri dish was covered with a
piece of glass. A small gap was kept for slow solvent evaporation at room
temperature overnight.
6. The membrane was then removed from the Petri dish. The cast film was dried in
an oven at 60oC for 24 hours without vacuum and another 24 hours with
vacuum. Then, the oven temperature was increased from 60 to 250°C at a
heating rate of 25°C/hour.
7. The membrane was annealed at 250°C for 24 hours in the vacuum oven, then
cooled down to room temperature naturally prior to the removal from the
vacuum oven.
28
Silane-modified SAPO-34/6FDA-durene MMM was synthesized by using the same
procedure as SAPO-34/6FDA-durene MMM fabrication. The amount of silane-modified
SAPO-34 crystals loaded was 5, 10, 15 and 20 wt% as well.
3.4.6 Characterization of Mixed Matrix Membrane
Mixed matrix membrane (MMM) was characterized by using the following methods:
1. The presence of SAPO-34 in the resulting MMM was verified by the pattern of
the X-ray Diffraction (XRD).
2. The functional groups and silane-modified grafting present in SAPO-34 crystals
were determined by using the Fourier Transform Infrared Spectroscopy (FTIR).
3. The morphology of MMM was studied by using the Scanning Electron
Microscopy (SEM).
4. The dispersion of fillers in the MMM was studied by using the Energy
Dispersive X-ray (EDX).
3.4.7 Gas Permeability and Selectivity Test
The performance of the resulting mixed matrix membrane (MMM) on CO2/CH4
separation were tested by using the CO2 membrane cell filter test rig (CO2MCEF) as
shown in Figure 6. The operating parameters were as follows:
Temperature: 25oC
Feed pressure: 5 bar
Feed composition: 100% CO2 and 100% CH4
29
Figure 6: CO2 membrane cell filter test rig (CO2MCEF)
The permeability of CO2 and CH4 were calculated by using equation 2 and 3,
respectively (Ismail & Lai, 2004).
---------- (2)
---------- (3)
is the permeability of CO2 in Barrer (1 Barrer = 1 x 10
-10 cm
3 (STP) cm/cm
2 s
cmHg), is the permeability of CH4 in Barrer,
is the volumetric flow rate of
CO2, is the volumetric flow rate of CH4, is the membrane skin thickness, is the
membrane effective surface area, while is the pressure difference across membrane.
The selectivity of CO2/CH4 was determined by the ratio of the permeability of CO2 and
CH4 as shown in equation 4 (Ismail & Lai, 2004).
(
) ---------- (4)
30
3.5 Project Activities and Key Milestones
The main activities of this research were to synthesis the membrane and to conduct the
membrane performance test towards gas separation. Inorganic fillers SAPO-34 crystals,
silane-modified SAPO-34 crystals as well as the polymer of 6FDA-durene were
synthesized prior to the fabrication of mixed matrix membrane (MMM). MMM was
fabricated by incorporating the inorganic fillers into the polymer matrix. The resulting
SAPO-34 and silane-modified SAPO-34 were characterized by using the X-ray
Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning
Electron Microscopy (SEM), whereas the fabricated MMM was characterized by using
the Energy Dispersive X-ray (EDX) and SEM. The performance of the MMM in
CO2/CH4 gas separation was evaluated by using the CO2 membrane cell filter test rig
(CO2MCEF) in order to determine the optimum loading of the inorganic particles into
the polymer phase. Table 4 shows the key milestones of the present research.
Table 4: Key milestones of the research
Progress Date of Completion
Confirmation of research topic 29th
May 2014
Problem identification, literature review and preparation
of experimental procedure for membrane synthesis 19
th June 2014
Synthesis of 6FDA-durene polymer 24th
July 2014
Synthesis of SAPO-34 crystals 26th
September 2014
Preparation of 6FDA-durene dense film 3rd
October 2014
Fabrication of SAPO-34/6FDA-durene MMM 3rd
October 2014
Preparation of silane-modified SAPO-34 crystals 10th
October 2014
Fabrication of silane-modified SAPO-34/6FDA-durene
MMM 17
th October 2014
Characterization of MMM 13th
November 2014
Gas permeability and selectivity test 21st November 2014
3.6 Gantt Chart
Table 5 shows the Gantt chart of the present research work.
31
Table 5: Gantt chart
No. Project Activities
Final Year Project 1 Final Year Project 2
Week
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14
1 Confirmation of project topic
2 Preliminary research work
3 Submission of extended proposal
4 Proposal defense
5 Project work continuous
6 Synthesis of 6FDA-durene polymer
7 Submission of interim draft report
8 Submission of interim report
9 Synthesis of SAPO-34 crystals
10 Preparation of 6FDA-durene dense film
11 Fabrication of SAPO-34/6FDA-durene mixed matrix membrane (MMM)
12 Preparation of silane-modified SAPO-34
crystals
13 Fabrication of silane-modified SAPO-34/6FDA-durene MMM
14 Characterization of MMM
15 Gas permeability and selectivity test
16 Submission of progress report
17 Pre-SEDEX
18 Submission of draft final report
19 Submission of dissertation (soft bound)
20 Submission of technical paper
21 Viva
22 Submission of project dissertation (hard
bound)
31
32
CHAPTER 4
RESULTS AND DISCUSSION
4.1 6FDA-Durene Polymer
6FDA-durene polymer was synthesized by using the chemical imidization method.
6FDA-dianhydride and durene-diamine monomers were purified prior to the synthesis
of 6FDA-durene polymer. Figure 7 and 8 show the purified 6FDA dianhydride and
durene-diamine monomers, respectively.
Figure 7: Purified 6FDA-dianhydride monomers
Figure 8: Purified durene-diamine monomers
33
Purified 6FDA-dianhydride and durene-diamine monomers were dissolved in purified
NMP solvent in order to prepare 21 wt% of monomers in solution. Polymerization was
carried out under nitrogen purge at room temperature to form polyamic acid (PAA). The
process was followed by chemical imidization by adding propionic anhydride (PA) and
triethylamine (TEA) to convert PAA to polyimide. Figure 9 shows the 6FDA-durene
polyimide formed. It was observed that the polyimide formed was viscous with light
yellowish appearance.
Figure 9: 6FDA-durene polyimide after synthesis
The 6FDA-durene polyimide produced was then precipitated with methanol. Methanol
was used as precipitating agent as it will dissolve the unreacted monomers after the
chemical imidization, and retain the 6FDA-durene polyimide structure. The polyimide
was washed with methanol for several times for solvent exchange before drying in the
vacuum oven at 150oC for 24 hours. Figure 10 shows the 6FDA-durene polyimide after
drying.
Figure 10: 6FDA-durene polyimide after drying
34
6FDA-durene polyimide was successfully synthesized. It was used as the polymer
matrix in the fabrication of mixed matrix membrane (MMM).
4.2 SAPO-34 Crystals
SAPO-34 crystals were synthesized by hydrothermal synthesis method as reported by
Askari & Halladj (2012). During the preparation of SAPO-34, the gel solution with
molar composition of Al2O3: P2O5: 0.6 SiO2: 2 TEAOH: 70 H2O was prepared before it
was transferred into the Teflon-lined synthesis reactor for hydrothermal growth. The
solid products which contained SAPO-34 particles were recovered and centrifuged with
deionized water upon completion of synthesis. Figure 11 shows the SAPO-34 crystals
before drying in the oven at 110oC overnight.
Figure 11: SAPO-34 crystals before drying
The dried crystals were calcined with air in the furnace to eliminate TEAOH template
from SAPO-34 framework. The final products of SAPO-34 crystals after calcination are
shown in Figure 12.
35
Figure 12: Final products of SAPO-34 crystals after calcination
The SEM image of SAPO-34 crystals is illustrated in Figure 13. It was observed that the
particles have a smooth external surfaces and displaying cubic shape morphology,
which is typical of SAPO-34 crystals. The crystals have an average size of ~2 µm.
Figure 13: SEM image of SAPO-34 crystals. The scale bar (8 µm) is represented by 4
grids
The XRD pattern of SAPO-34 crystals is shown in Figure 14. The XRD pattern
exhibited the identical characteristic peaks at 2-theta values of 9.5o, 15.2
o and 20.8
o
approximately, which correspond to the standard XRD pattern of SAPO-34 crystals as
reported by Robson & Lillerud (2001), confirming the formation of SAPO-34 phase.
36
Figure 14: XRD pattern of synthesized SAPO-34 crystals
The synthesized SAPO-34 crystals were then used as the inorganic phase to fabricate the
mixed matrix membrane (MMM).
4.3 Silane-Modified SAPO-34 Crystals
Silane-modified SAPO-34 crystals were prepared by mixing the silane coupling agent
(APTES) and toluene with the as-synthesized SAPO-34 crystals. The mixture was
stirred under nitrogen purge as shown in Figure 15.
Figure 15: Stirring of silane-modified SAPO-34 crystals mixture under nitrogen purge
5 10 15 20 25 30 35 40 45 50
Inte
nsi
ty (
a.u
.)
2-Theta (°)
(001)
(111)
(102)
37
The silane-modified crystals were filtered and washed with toluene and methanol,
respectively to remove the unreacted silane before drying in the oven.
Figure 16: FTIR spectra of SAPO-34 and silane-modified SAPO-34 crystals
Figure 16 shows the FTIR spectra of SAPO-34 and silane-modified SAPO-34 crystals.
The significant band between the wavenumber of 900-1350 cm-1
in the FTIR spectra for
both samples indicates the asymmetric vibration modes of Si-O and Al-O group in
SAPO-34. A band of N-H at around 1450-1550 cm-1
was observed in the spectra of
silane-modified SAPO-34. This result shows that the silane group was successfully
grafted onto the SAPO-34 structure.
4.4 Mixed Matrix Membranes
Mixed matrix membranes (MMMs) were fabricated by incorporating different loadings
of SAPO-34 inorganic fillers in 6FDA-durene polyimide. In the present research, 5, 10,
15 and 20 wt% of SAPO-34 and silane-modified SAPO-34 crystals were successfully
added into the 6FDA-durene polymer matrix to synthesis total of eight MMMs (Four
SAPO-34/6FDA-durene MMMs and four silane-modified SAPO-34/6FDA-durene
MMMs). Pure 6FDA-durene membrane was fabricated as a reference membrane. Prior
5001000150020002500300035004000
Tran
smit
tan
ce (
%)
Wavenumber (cm-1)
SAPO-34
Silane-Modified SAPO-34
N-H
Al-O, Si-O
38
to membrane fabrication, 6FDA-durene polymer, calcined SAPO-34 and silane-
modified SAPO-34 crystals were dried at 110oC overnight to remove excess moisture.
During the fabrication of MMM, SAPO-34 dispersion and polymer solution were
prepared separately before mixing. The mixture was then stirred and sonicated for 2
hours alternately by using the priming method. Figure 17 shows the sonication process
of SAPO-34/6FDA-durene mixture in the sonicator.
Figure 17: Sonication of SAPO-34/6FDA-durene mixture in sonicator
After the mixture was further stirred for 2 hours, it was casted on Petri dish and covered
with a glass plate overnight for slow solvent evaporation as illustrated in Figure 18.
Figure 18: Slow solvent evaporation of casted membrane
After the resulting MMM was peeled off from the Petri dish, it was then dried and
annealed in the vacuum oven.
39
Figure 19 shows the fabricated SAPO-34/6FDA-durene MMMs, while Figure 20 shows
the fabricated silane-modified SAPO-34/6FDA-durene MMMs with different loadings
of SAPO-34 fillers.
Figure 19: SAPO-34/6FDA-durene mixed matrix membranes with 0, 5, 10, 15 and 20
wt% SAPO-34 loadings
Figure 20: Silane-modified SAPO-34/6FDA-durene mixed matrix membranes with 0, 5,
10, 15 and 20 wt% silane-modified SAPO-34 loadings
0 wt% 5 wt% 10 wt% 15 wt% 20 wt%
0 wt% S5 wt% S10 wt% S15 wt% S20 wt%
40
From Figure 19 and 20, it was observed that the fabricated MMMs were transparent and
possessed good mechanical property. There was no crack on the resulting MMMs and
with ductile and bendable property. The transparency characteristic of silane-modified
SAPO-34/6FDA-durene MMMs was found better than SAPO-34/6FDA-durene MMMs.
It might be due to the modification effect of APTES on the surface properties of SAPO-
34. In general, all fabricated MMMs have good physical appearance and mechanical
property.
4.5 Characterization of Mixed Matrix Membrane
4.5.1 Scanning Electron Microscopy
The morphology of the resulting mixed matrix membrane (MMM) was studied by using
the Scanning Electron Microscopy (SEM). Figure 21 and 22 depict the SEM images of
cross-section view of 6FDA-durene dense film and MMMs with and without silane-
modification of 5, 10, 15 and 20 wt% SAPO-34 fillers. The thickness of all membranes
was between 35-45 m.
Figure 21: Cross-section SEM image of pure 6FDA-durene membrane
Pure 6FDA-durene membrane
41
Figure 22: Comparison of cross-section SEM images of SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-durene
MMMs. (5, 10, 15 and 20) and (S5, S10, S15 and S20) represent the loadings of SAPO-34 and silane-modified SAPO-34 in wt%,
respectively
SAPO-34/6FDA-durene MMM (5) SAPO-34/6FDA-durene MMM (10) SAPO-34/6FDA-durene MMM (15) SAPO-34/6FDA-durene MMM (20)
Silane-modified SAPO-34/6FDA-
durene MMM (S5)
Silane-modified SAPO-34/6FDA-
durene MMM (S10)
Silane-modified SAPO-34/6FDA-
durene MMM (S15)
Silane-modified SAPO-34/6FDA-
durene MMM (S20)
42
The purpose of introducing the silane coupling agent on the surface of SAPO-34 is to
improve the compatibility between the polymeric and inorganic phases. From the SEM
images as shown in Figure 22, it was observed that the interfacial void or defect was
improved between the polymeric and inorganic phases for silane-modified SAPO-
34/6FDA-durene MMMs as compared to the SAPO-34/6FDA-durene MMMs. The filler
particles were detached to the polymer matrix without forming significant phase
separation between the two boundaries.
43
4.5.2 Energy Dispersive X-ray
Energy Dispersive X-ray (EDX) was carried out to study the dispersion of SAPO-34
fillers in the mixed matrix membrane (MMM). Figure 23 shows the EDX data of the
pure 6FDA-durene membrane, SAPO-34/6FDA-durene MMM loaded with 15 wt%
SAPO-34 and silane-modified SAPO-34/6FDA-durene MMM loaded with 15 wt%
silane-modified SAPO-34.
Element At%
C 47.2
F 28.8
O 20.8 N 3.2
Element At%
C 45.3 F 29.2
O 21.0
N 3.4 Al 0.6
Si 0.3
P 0.2
Element At%
C 44.7 F 29.2
O 20.7
N 3.8 Al 0.9
Si 0.4
P 0.3
Figure 23: EDX data of pure 6FDA-durene membrane, SAPO-34/6FDA-durene and
silane-modified SAPO-34/6FDA-durene MMM. (15) and (S15) represent the loadings
of SAPO-34 and silane-modified SAPO-34 in wt%, respectively
The main elements that presence in pure 6FDA-durene are carbon (C), fluorine (F),
oxygen (O) and nitrogen (N), while SAPO-34 contains the elements of aluminium (Al),
silicon (Si) and phosphorus (P). The amount of each element was presented in atomic
percentage (at%). From Figure 23, it was verified that pure 6FDA-durene membrane
contains only C, F, O and N with the amount of 47.2, 28.8, 20.8 and 3.2 atomic %,
respectively. For SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-durene
MMMs, it was verified that the elements of SAPO-34 (Al, Si and P) were presence in
the resulting MMMs.
Figure 24 displays the EDX mapping for SAPO-34/6FDA-durene MMM loaded with 15
wt% SAPO-34, while Figure 25 displays the mapping for silane-modified SAPO-
Pure 6FDA-durene membrane SAPO-34/6FDA-durene MMM (15) Silane-modified SAPO-34/6FDA-
durene MMM (S15)
44
34/6FDA-durene MMM loaded with 15 wt% silane-modified SAPO-34. The images in
both figures showed that SAPO-34 particles were uniformly dispersed in 6FDA-durene
polymer matrix. No agglomeration and phase separation was found in both MMMs.
46
Figure 25: EDX mapping of silane-modified SAPO-34/6FDA-durene MMM loaded with 15 wt% silane-modified SAPO-34
47
4.6 Gas Separation Performance
In the present research, the permeability of the carbon dioxide (CO2) and methane
(CH4), and CO2/CH4 selectivity of the resulting mixed matrix membranes (MMMs)
were determined. The membranes were tested in the CO2 membrane cell filter test rig
(CO2MCEF) at pressure of 5 bar and room temperature of 25oC. The results of the
separation performance were summarized in Table 6.
Table 6: Permeability and CO2/CH4 separation selectivity at 25oC and 5 bar
Mixed Matrix Membrane (MMM)
Permeability CO2/CH4
Ideal
Selectivity Carbon Dioxide,
CO2 (Barrer)
Methane, CH4
(Barrer)
6FDA-durene 408.23 18.71 21.82
5 wt% SAPO-34 in 6FDA-durene 239.50 23.28 10.29
10 wt% SAPO-34 in 6FDA-durene 206.97 21.49 9.63
15 wt% SAPO-34 in 6FDA-durene 251.32 26.10 9.63
20 wt% SAPO-34 in 6FDA-durene 217.72 24.95 8.73
5 wt% silane-modified SAPO-34
in 6FDA-durene 183.61 16.39 11.20
10 wt% silane-modified SAPO-34
in 6FDA-durene 166.32 15.35 10.84
15 wt% silane-modified SAPO-34
in 6FDA-durene 243.93 24.39 10.00
20 wt% silane-modified SAPO-34
in 6FDA-durene 235.87 24.95 9.45
For the pure 6FDA-durene membrane, the results of the gas permeability and the
selectivity of CO2/CH4 separation are comparable to the results reported by Liu, Chng,
Chung, & Wang (2003). However, it was found that the permeability of CO2 and
CO2/CH4 selectivity of the resulting MMMs decrease significantly as compared to the
pure 6FDA-durene membrane. Several justifications are made to discuss the possible
causes of the abovementioned results:
1. Large inorganic particles size and moisture contact penetrated the gas diffusion.
Typical SAPO-34 particles have a bigger size (~ 2 m) as compared to other
inorganic particles. Due to the large size of SAPO-34, sedimentation of particles
might occur during the fabrication of MMM. However, the effect of large
48
particles size seems to be less towards the gas separation performance of MMM.
Different particles size might be the factor for the variation of permeability and
selectivity results. Besides, the resulting MMM might get contact with the air
moisture in the surrounding before conducting the gas permeability and
CO2/CH4 selectivity study. Gas molecules were unable to penetrate through the
pores of SAPO-34 due to the pores blockage, and thus degraded the performance
of the MMM.
2. Poor interfacial adhesion created interfacial void. Even though the morphology
of the resulting MMMs showed the improvement on the compatibility between
the polymeric and inorganic phases, there might still have some minor interface
voids existed in the MMM. Gas molecules could easily penetrate into the
membrane via the interfacial void generated because void provides the path with
the least resistance. Thus, it will deteriorate the selectivity of the resulting
membranes.
3. Formation of rigidified polymer layer at SAPO-34/6FDA-durene interphase
(matrix rigidification). Rigidified polymer might form in the resulting MMM
and caused the immobilization of polymer chains at SAPO-34/6FDA-durene
interphase, which will reduce the gas sorption and permeation (Manson & Chiu,
1973). The formation of matrix rigidification is illustrated in Figure 26 (A).
Rigidified polymer might seal the SAPO-34 pores and caused the pores
blockage, which is known as “plugged sieves” (Rezakazemi, Amooghin,
Rahmati, Ismail, & Matsuura, 2014), as demonstrated in Figure 26 (B). Thus, the
gas permeability and selectivity would be affected.
49
Figure 26: Schematic diagram of (A) matrix rigidification and (B) plugged sieves
(Rezakazemi, Amooghin, Rahmati, Ismail, & Matsuura, 2014)
Besides, there was no significant improvement on the CO2/CH4 selectivity of the
resulting MMMs after introducing the silane coupling agent (APTES) in the inorganic
phase. Theoretically, the application of silane group should improve the interfacial
strength between the polymeric and inorganic phases, and thus enhance the membrane
selectivity. Therefore in this context, the selection of appropriate silane group and its
effect towards the separation performance of MMM is an important consideration in
fabricating a chemical-modified membrane.
(A) (B)
Filler Filler
Rigidified
polymer
50
CHAPTER 5
CONCLUSION AND RECOMMENDATION
5.1 Conclusion
In the present research, 6FDA-durene polymer, SAPO-34 and silane-modified SAPO-34
crystals were successfully synthesized prior to the fabrication of mixed matrix
membrane (MMM). 6FDA-durene polymer was synthesized by chemical imidization
method while SAPO-34 crystals were synthesized by hydrothermal synthesis method.
SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-durene MMMs were
successfully fabricated by incorporating different loadings (5, 10, 15 and 20 wt%) of
inorganic fillers in polymer matrix. The MMMs fabricated were transparent and showed
good mechanical property in which they were ductile and bendable. No crack was found
on all fabricated MMMs.
The formation of SAPO-34 phase was verified by the SEM morphology and XRD
pattern. APTES was successfully used as the silane coupling agent to prepare the silane-
modified SAPO-34 crystals. The FTIR spectra showed that silane group was
successfully grafted into the SAPO-34 structure. The cross-section SEM morphology of
the fabricated MMMs generally showed the improvement on the compatibility between
the polymeric and inorganic phases. The formation of interfacial voids and defects were
reduced in the silane-modified SAPO-34/6FDA-durene MMMs as compared to the
SAPO-34/6FDA-durene MMMs. No significant phase separation was formed between
the two boundaries. The objective to improve the compatibility between the two phases
through silane-modification on the surface of the inorganic fillers was successfully
achieved. EDX results showed that inorganic SAPO-34 was well distributed in the
polymer matrix. No agglomeration was found in the EDX mapping for both SAPO-
34/6FDA-durene and silane-modified SAPO-34/6FDA-durene MMMs. EDX results
also verified the presence of SAPO-34 in the resulting MMMs. The amount of each
element in the membranes was presented in atomic %.
51
In this research, the gas separation performance of the resulting MMMs was
successfully conducted by using the CO2 membrane cell filter test rig (CO2MCEF).
However, the permeability of CO2 and CO2/CH4 selectivity of the resulting MMMs
decreased significantly for all the resulting MMMs, compared to the pure 6FDA-durene
membrane. Several possible causes such as large particles size of SAPO-34, moisture
contact with MMM, poor interfacial contact between the polymeric and inorganic
phases, and polymer matrix rigidification were justified.
In general, all fabricated SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-
durene MMMs exhibited excellent characterization results. Modifications and process
optimization are needed in later works to improve the performances of MMM in gas
permeability and CO2/CH4 separation selectivity.
5.2 Recommendation
There are several recommendations and modifications that can be done in future to
improve the synthesizing processes, characterization results and gas separation
performances of SAPO-34/6FDA-durene and silane-modified SAPO-34/6FDA-durene
mixed matrix membranes (MMMs):
1. Priming process of polymer and SAPO-34 solutions mixture. The time allocated
for stirring and sonication of the polymer and SAPO-34 solutions mixture can be
adjusted or extended in order to achieve better priming process (obtain good
dispersion between the two solutions).
2. Gas permeability and CO2/CH4 selectivity test. The MMM is suggested to be
heated in the vacuum oven at 110oC overnight to remove the excess moisture
that might contain in the MMM. The molding of MMM before putting into the
test rig must be handled more carefully in order to avoid any defect and moisture
contact to the MMM. Besides, it is encouraged to vacuum the molded membrane
overnight after putting it into the test rig to remove the gas molecules or
impurities that might attach to the MMM, in order to enhance its gas separation
performance.
52
3. Selection of appropriate silane group. The selection of suitable silane group is
the main concern for the preparation of silane-modified inorganic fillers. Other
types of silane coupling agents such as methoxy silanes: 3-
Aminopropyltrimethoxysilane (APTMS), N-(2aminoethyl)-3-aminopropyl-
trimethoxysilane (AAPTMS) and other amine agents can be used instead of
APTES to study their effects in inorganic phase towards the gas separation
performance of MMM. This is because different silane group will graft
differently with inorganic phase and exhibits different performance results.
According to Junaidi, Khoo, Leo, & Ahmad (2014), methoxy silane is more
readily in hydrolysis reaction compared to ethoxy group.
4. Optimum loading of inorganic fillers. The loadings of 5 to 20 wt% of SAPO-34
and silane-modified SAPO-34 in 6FDA-durene polymer matrix might be too
high for the fabrication of SAPO-34/6FDA-durene and silane-modified SAPO-
34/6FDA-durene MMMs. Therefore, a lower loading of 1 to 3 wt% of SAPO-34
and silane-modified SAPO-34 can be considered for future fabrication of these
types of membranes.
5. Characterization of MMM. The resulting membrane can be further characterized
by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry
(DSC). TGA is used to study the thermal stability while DSC is used to study the
amount of heat required to increase the temperature of a sample.
53
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